While Ziegler-Natta catalysts are a mainstay for polyolefin manufacture, single-site (metallocene and non-metallocene) catalysts represent the industry's future. These catalysts are often more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include reduced low molecular weight extractables, enhanced incorporation of α-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics. Single-site catalysts typically give narrow molecular weight distribution, which can improve some properties but often is detrimental to processability.
Single-site olefin polymerization catalysts having “open architecture” are generally known. Examples include the so-called “constrained geometry” catalysts developed by scientists at Dow Chemical Company (see, e.g., U.S. Pat. No. 5,064,802), which have been used to produce a variety of polyolefins. “Open architecture” catalysts differ structurally from ordinary bridged metallocenes, which have a bridged pair of pi-electron donors. In open architecture catalysts, only one group of the bridged ligand donates pi electrons to the metal; the other group is sigma bonded to the metal. An advantage of this type of bridging is thought to be a more open or exposed locus for olefin complexation and chain propagation when the complex becomes catalytically active. Simple examples of complexes with open architecture are tert-butylamido(cyclopentadienyl)dimethylsilyl-zirconium dichloride and methylamido(cyclopentadienyl)-1,2-ethanediyl-titanium dimethyl: 
Organometallic complexes that incorporate “indenoindolyl” ligands are known (see U.S. Pat. No. 6,232,260 and PCT Int. Appl. WO 99/24446 (“Nifant'ev”)). The '260 patent demonstrates the use of non-bridged bis(indenoindolyl) complexes for making HDPE in a slurry polymerization. Versatility is an advantage of the complexes; by modifying the starting materials, a wide variety of indenoindolyl complexes can be prepared. “Open architecture” complexes are neither prepared nor specifically discussed. Nifant'ev teaches the use of bridged indenoindolyl complexes as catalysts for making polyolefins, including polypropylene, HDPE, and LLDPE. The complexes disclosed by Nifant'ev do not have open architecture.
PCT Int. Appl. WO 01/53360 (Resconi et al.) discloses indeno[2,1-b]indolyl complexes having open architecture and their use to produce substantially amorphous propylene-based polymers. There are no measurements of molecular weight distribution of these propylene polymers and there is no indication that indeno[2,1-b]indolyl complexes having open architecture can be used to produce polyethylene or polypropylene with broad molecular weight distribution.
U.S. Pat. No. 6,559,251 discloses a process for copolymerizing ethylene with at least one alpha-olefin selected from the group consisting of 1-butene, 1-hexene, and 1-octene in the presence of a catalyst system which comprises an activator and a silica-supported, indenoindolyl Group 4-6 transition metal complex having open architecture to produce an ethylene copolymer having a density less than about 0.910 g/cm3. While both indeno[1,2-b]indolyl and indeno[2,1-b]indolyl open architecture complexes are disclosed, no comparative results are given. Nor is there any indication that indeno[2,1-b]indolyl complexes having open architecture can be used to produce polyethylene with broad molecular weight distribution. The molecular weight distributions reported in the examples are all narrow, varying from 2.8 to 3.1.
Pending application Ser. No. 10/638,592 filed Aug. 11, 2003 discloses a process for polymerizing ethylene with open architecture complexes containing an indenoindolyl ligand linked to a C6–C20 alkylamido ligand. The only polymerization results reported are with indeno[1,2-b]indolyl complexes, and there is no indication that indeno[2,1-b]indolyl complexes having open architecture can be used to produce polyethylene with broad molecular weight distribution.
Despite the considerable work done in this area, there is much that is not understood. There is a continued need for improved catalysts for ethylene polymerizations. In particular, there is a need for a process that uses single-site catalysts but still makes polyethylenes with broad molecular weight distribution and correspondingly good processability.
U.S. Pat. No. 6,479,609 teaches the use of a multi-stage process and single-site catalysts to make polyethylene with broad molecular weight distribution. This process is complicated and requires more equipment than a standard polymerization. They note that prior art taught mixtures of catalytic systems in a one-stage polymerization but that this has many drawbacks. In particular, the catalyst feed rate is difficult to control and the polymer particles produced are not uniform in size; segregation of the polymer during storage and transfer usually produces non-homogeneous products. Thus, there remains a need for a simple process to make polyethylene with broad molecular weight distribution.